GCSE WJEC Biology Notes - Cells and Membrane Movement

Cell Structure and Organisation

You're basically a walking collection of 37 trillion cells! These tiny building blocks come in two main types: animal cells and plant cells. Both share some key components, but plants have a few extras.

Animal cells contain four essential parts you need to know. The nucleus acts like the cell's control centre, housing chromosomes that carry genetic information. Mitochondria are the powerhouses where aerobic respiration happens, providing energy for everything the cell does. The cell membrane works as a selective bouncer, controlling what enters and exits. Finally, cytoplasm is the gel-like substance where most chemical reactions occur.

Plant cells have everything animal cells do, plus some unique features. Chloroplasts contain chlorophyll and carry out photosynthesis - basically how plants make their own food using sunlight. The rigid cell wall made of cellulose gives plants their structure and strength, which is why trees don't collapse!

Quick Tip: Remember that plant cells are basically animal cells with extra green superpowers - chloroplasts for photosynthesis and cell walls for strength!

Microscopes and Magnification

Want to see cells up close? You'll need a microscope! There are two main types, each with their own superpowers and limitations.

Light microscopes use light passing through specimens and let you observe living cells in colour. They're perfect for watching cells in action, but they have limited magnification power. Electron microscopes fire beams of electrons instead of light, giving you incredibly detailed black and white images with much higher magnification and resolution - but only dead specimens.

Here's the magnification formula you absolutely need to know: Total magnification = eyepiece lens power × objective lens power. For example, if your eyepiece is ×10 and objective is ×40, your total magnification is ×400.

You can also calculate magnification using: Magnification = Image size ÷ Real size. Just make sure both measurements use the same units!

Exam Hack: The magnification formula is guaranteed to appear on your exam - practise it until you can do it in your sleep!

Cell Organisation - From Cells to Systems

Your body is organised like a brilliant hierarchy, starting small and building up to create amazing organ systems. Think of it as nature's ultimate teamwork project!

Similar cells group together to form tissues. Muscular tissue contracts to create movement wherever it's attached, while glandular tissue produces and secretes important chemicals like enzymes and hormones.

Different tissues combine to create organs. Your stomach is a perfect example - it uses muscular tissue to churn food around and glandular tissue to produce digestive juices that break down your dinner.

Multiple organs work together as organ systems. The digestive system includes your stomach, liver, small intestine, and large intestine all collaborating to process food. Finally, all your organ systems work together to create you - a complete living organism!

Memory Trick: Remember the hierarchy as "Cells → Tissues → Organs → Organ Systems → Organism" - it's like building with biological Lego blocks!

Diffusion and Cell Membrane Transport

Cells need to constantly move substances in and out, and diffusion is one of the key ways this happens. It's basically particles moving from crowded areas to less crowded ones - like people spreading out in a busy corridor!

Diffusion is the net movement of particles from an area of higher concentration to lower concentration. This movement down the concentration gradient happens naturally in liquids and gases because particles can move around freely.

Your cell membrane is selectively permeable, meaning it has tiny holes that only let small particles like glucose and amino acids pass through. Large molecules like starch and proteins are too big to fit - think of it as a very exclusive club with strict size limits!

The brilliant thing about diffusion is that it's passive - it doesn't require any energy from cells to happen. This is crucial for oxygen and carbon dioxide movement during aerobic respiration.

Real-Life Connection: Diffusion is why you can smell someone's perfume from across the room - the scent molecules naturally spread out through the air!

Osmosis and Active Transport

Osmosis is a special type of diffusion that only involves water molecules. It's the net movement of water across a selectively permeable membrane from areas with high water concentration to low water concentration.

Think of it this way: where there are more dissolved substances, there's less space for water molecules. Water naturally flows to balance things out, creating a steady flow until equilibrium is reached.

Active transport is completely different - it's like going uphill instead of downhill! Particles move against the concentration gradient, which requires energy. Cells that do lots of active transport have more mitochondria to provide this energy.

Active transport is essential in two key areas. In your gut, it absorbs sugar molecules into your blood even when blood sugar levels are already high. In plant roots, root hairs use it to absorb mineral ions from soil, even when mineral concentration is lower in the soil than in the plant.

Energy Alert: Remember that diffusion and osmosis are free (passive), but active transport costs energy - just like walking downhill versus climbing uphill!

Enzymes - Biological Catalysts

Enzymes are the ultimate biological helpers - proteins made by living cells that massively speed up chemical reactions. Without these biological catalysts, your body's reactions would be so slow you literally couldn't survive!

Here's how they work: the substrate (the chemical that needs changing) must fit perfectly into the enzyme's active site. Think of it like a lock and key - only the right substrate fits the right enzyme to form an enzyme-substrate complex.

This specificity is crucial. Each enzyme only works with particular substrates because the active site has a unique shape. Once the substrate binds, the enzyme helps the reaction happen much faster, then releases the products and is ready to work again.

The amazing thing about enzymes is that they don't get used up in reactions - they're like the perfect recycling system! One enzyme molecule can catalyse thousands of reactions.

Key Concept: The lock-and-key model shows why enzymes are so specific - if the substrate doesn't fit the active site perfectly, no reaction occurs!

Enzyme Structure and Function

Enzymes are proteins built from smaller units called amino acids. These amino acids link together in chains, and different proteins contain different sequences of amino acids.

The order of amino acids in the chain is absolutely crucial because it determines how the protein folds into its final 3D shape. This shape directly affects the active site - change the shape even slightly, and the enzyme might stop working completely.

This explains why enzymes are so sensitive to changes in temperature and pH. Extreme conditions can alter the protein's shape, denaturing the enzyme and destroying its ability to function.

Understanding enzyme structure helps explain their incredible specificity and efficiency. The precise 3D arrangement creates the perfect environment for speeding up specific chemical reactions.

Structure = Function: In biology, the shape of something almost always determines what job it can do - this is especially true for enzymes!